“If we can flip Jeff Sessions’s seat, it is going to be good for the entire Democratic Party, and it’s going to be good for the country because people will start to see there’s some rejection of what’s going on,” Mr. Jones said, inviting Democrats to take the race seriously while being careful not to pointedly criticize Mr. Trump, who remains popular in Alabama.

Mr. Jones, who as a United States attorney in the 1990s prosecuted the Birmingham church bombers, has the support of an array of Alabama Democrats and national party figures, including former Vice President Joseph R. Biden Jr. and Representative John Lewis of Georgia.

Most Republicans are highly skeptical that a Democrat could win a Senate race in deeply conservative Alabama. They do not believe the seat would be in jeopardy, even in a special election featuring Mr. Moore, a polarizing figure who was dismissed from the bench in 2003 for refusing to remove a Ten Commandments monument from courthouse grounds.

Yet establishment-aligned Republicans are alarmed at the prospect that he could be their standard-bearer. At a conference sponsored by the Business Council of Alabama on the state’s Gulf Coast this weekend, the legislators and lobbyists in attendance nervously talked about how close Mr. Moore could come to winning outright Tuesday.

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Roy S. Moore, a deposed State Supreme Court justice who is also vying for the Republican nomination, met with pastors in Piedmont, Ala., last week.Credit
Kevin D. Liles for The New York Times

And Mr. Strange’s supporters, previewing a message they will amplify in the case of a runoff between the sitting senator and Mr. Moore, are already warning that nominating the former judge will carry at least some risk.

“National Democrats are itching for a fight that will embarrass President Trump,” said Steven Law, who runs the main group backing Mr. Strange, the Senate Leadership Fund, which has been blanketing Alabama’s airwaves.

Yet the ad campaign by the McConnell-aligned Senate Leadership Fund — while doing significant damage to a third Republican candidate in the race, Representative Mo Brooks, a hard-line conservative — has also highlighted Mr. Strange’s ties to the congressional Republican leadership. And it has come at a moment when conservatives are furious at lawmakers for not repealing the Affordable Care Act and Mr. Trump is placing much of the blame at the feet of Mr. McConnell.

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Naturally, Mr. Strange’s rivals have seized on his ties to Mr. McConnell. At a candidate forum in nearby Trussville this month, one of Mr. Brooks’s top applause lines was a call for Mr. McConnell to be removed as the party’s Senate leader.

In an interview near his campaign bus, a “Ditch Mitch” banner fastened to the front, Mr. Brooks even found a way to invoke Mr. McConnell when pressed about Mr. Trump’s decision to endorse Mr. Strange.

“I sure hope that the president was getting bad advice from whoever it is — Mitch McConnell and others — that persuaded him to make the Luther Strange endorsement,” Mr. Brooks said after yet another joint appearance with the other candidates. (In a more candid moment, Mr. Brooks conceded that he “knew that there would be potential consequences” for his decision to stand up for Mr. Sessions when the president was assailing his own attorney general.)

Mr. Moore, in a separate interview, spoke less directly about Mr. Trump and Mr. McConnell, but said the national party may be in for a surprise.

“It doesn’t always work the way Washington thinks it works,” Mr. Moore said after he gathered with about two dozen pastors over Chick-fil-A and doughnuts. “They’re on a different track than the people of Alabama.”

For his part, Mr. Strange dismissed the criticism over his endorsements and pointed to his record as Alabama’s attorney general.

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A third Republican candidate in the race, Representative Mo Brooks, onstage at the party gathering in Homewood. A hard-line conservative, Mr. Brooks has highlighted Mr. Strange’s ties to the congressional party leadership.Credit
Kevin D. Liles for The New York Times

“What distinguishes me from my opponents is I actually have a record of conservative, common-sense accomplishments as opposed to just the rhetoric about, ‘I’m a conservative guy, I love President Trump,’” he told reporters in this Birmingham suburb last week.

Yet Mr. Strange’s other, perhaps graver challenge, besides carrying the Washington establishment mantle, is voter concern about that same record in Alabama’s capital.

“A lot of people were very frustrated by the appointment of Luther Strange by our former governor,” said Sharon Denham, the president of the North Jefferson County Republican Club.

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Mr. Bentley resigned in April after pleading guilty to two misdemeanor charges for improper handling of campaign funds. The governor, whose scandal grew out of a cover-up of an improper relationship with a close aide, elevated Mr. Strange to the Senate in February.

Mr. Strange, as the state attorney general, was overseeing an investigation into Mr. Bentley’s conduct at the time. Both men have denied that Mr. Strange’s handling of the inquiry influenced the Senate appointment. But their defenses have seemingly done little to diminish the skepticism that has shadowed Mr. Strange’s campaign.

“There was a lot of smoke there and people didn’t take to that very kindly,” said Reed Ingram, a Republican state representative who has not taken sides in the Senate race.

Mr. Strange’s campaign hoped that Mr. Trump’s endorsement, bestowed via Twitter last week, would move the conversation away from Mr. McConnell and Mr. Bentley. And both the senator and his allies, including a major pro-Trump “super PAC,” have bought broadcast and digital ads to draw attention to the president’s intervention in the race.

In a 10-person field and low-turnout summer primary, Mr. Trump’s endorsement may be enough to stir sufficient voter interest and keep Mr. Moore below 50 percent while lifting Mr. Strange into a runoff.

Still, just how many votes the endorsement will change in a divided state party is uncertain.

“I don’t know if there is such a thing as party unity within the Republican Party,” Ms. Denham, the president of a local Republican club, said. “That’s part of what makes us Republicans: that desire to think for ourselves and not be told what to do or how to think by somebody else.”

Or as Mr. Ingram put it: “Alabama is kicked by a different mule and we just don’t like to be told who to vote for.”

Correction: August 13, 2017

An earlier version of this article misstated the last year Republicans lost a Senate race in Alabama. It was 1992, not 1990.